This application relates to optical fiber devices, and more specifically, to devices based on evanescent optical coupling between two fibers.
Optical waves may be transported through optical waveguiding elements or xe2x80x9clight pipesxe2x80x9d such as optical fibers. A typical fiber may be simplified as a fiber core and a cladding layer surrounding the fiber core. The refractive index of the fiber core is higher than that of the fiber cladding to confine the light. Light rays that are coupled into the fiber core within a maximum angle with respect to the axis of the fiber core are totally reflected at the interface of the fiber core and the cladding. This total internal reflection provides a mechanism for spatially confining the optical energy of the light rays in one or more selected fiber modes to guide the optical energy along the fiber core. Optical fibers may be used in transmission and delivery of optical signals from one location to another in a variety of optical systems, including but not limited to, fiber devices, fiber links and fiber networks for data communications and telecommunications. In addition, optical fibers may be used to form various optical devices to modify, filter, or process guided optical energy.
The guided optical energy in a fiber, however, is not completely confined within the core of the fiber. A portion of the optical energy can xe2x80x9cleakxe2x80x9d through the interface between the fiber core and the cladding via an evanescent field that essentially decays exponentially with the distance from the core-cladding interface. The distance for a decay in the electric field of the guided light by a factor of e≈2.718 is about one wavelength of the guided optical energy. This evanescent leakage may be used to couple optical energy into or out of the fiber core, or alternatively, to perturb the guided optical energy in the fiber core.
According to one embodiment, a fiber device includes a substrate having first and second opposing substrate surfaces and including an elongated groove formed over the first substrate surface, and a fiber having a fiber coupling portion engaged in the elongated groove. A portion of fiber cladding of the fiber coupling portion is removed to form a side fiber surface spaced from a fiber core of the fiber within a reach of an evanescent field of a guided mode in the fiber. The fiber coupling portion has a fiber cladding portion whose radial index distribution is different from adjacent fiber portions to produce a radial mode profile wider than a radial mode profile of the adjacent fiber portions.
Methods for fabricating a fiber device are also provided. In one embodiment, a method includes selecting a fiber portion of a fiber, removing a portion of fiber cladding of the selected fiber portion to form a fiber coupling surface spaced from a fiber core of the selected fiber portion within a reach of an evanescent field of a guided mode, and modifying a property of said selected fiber portion. The modification increases a cladding refractive index of the selected fiber portion and to increase a spatial mode profile of said guided mode in the selected fiber portion to be greater than a spatial mode profile of the guided mode in an adjacent fiber portion whose cladding refractive index is not modified.